This invention relates to a cathode-ray tube for use in color image display. More in particular, in an in-line type electron gun assembly for arranging and emitting three electron beams in a lateral direction toward a fluorescent screen or a phosphor screen, the present invention relates to an improvement in the shapes of electron beam passage holes in a plurality of electrode units constituting an electrostatic lens or an electron lens.
Conventionally, an in-line type electron gun assembly equipped with a main electron lens employs the structure described, for example, in JP-A-58-103752 (corresponding to U.S. patent Specification No. 4,581,560 assigned to the assignee of the present invention and issued on Apr. 8, 1986).
FIGS. 1a, 1b and 1c are structural views showing a schematic structure of the in-line type electron gun assembly equivalent to the one described in the reference. FIG. 1a is a longitudinal sectional view in a vertical direction, FIG. 1b is a transverse sectional view taken along a line 1b--1b in FIG. 1a in a horizontal direction, and FIG. 1c is a transverse sectional view taken along a line 1c--1c in FIG. 1a in a horizontal direction.
In FIG. 1a, reference numeral 41 denotes a cathode for generating three electron beams; 42 a G1 electrode for limiting the diameters of the electron beams emitted from the cathode 41; 43 a G2 electrode for accelerating the electron beams from the G1 electrode 42; and 43 a cylindrical G3 electrode for accelerating and converging the electron beams from the G2 electrode 43, having an aperture of a substantially elliptic cross section. A plate member 46 is disposed on the inner wall of a cylindrical member of the G3 electrode 44, and another plate member is disposed on the inner wall of a cylindrical member of the G4 electrode 45.
The G3 electrode 44 equipped with the plate member 46 and the G4 electrode equipped with the plate member 47 together constitute the main electron lens. Three, substantially elliptic electron beam passage holes are formed in each of the plate members 46 and 47 of the G3 and G4 electrodes, respectively, as shown in FIGS. 1b and 1c. These three substantially elliptic electron beam passage holes are so constituted that the diameter (the major diameter) in their lengthwise direction is equal to one another.
The in-line type electron gun having such a construction operates in the following way.
Thermoelectrons emitted from the three cathodes 41 heated by a heater (not shown) are attracted to the G1 electrode 42 by a positive voltage applied to the G2 electrode 43 and form three electron beams. These three electron beams pass through the electron beam passage holes of the G1 electrode 42 and then through the electron beam passage holes of the G2 electrode 43 and thereafter enter the main electron lens constituted by the electrodes 44 and 45 while being accelerated by the positive voltages applied to the G3 and G4 electrodes 44 and 45. A low voltage of from about 5 to about 10 KV is supplied to the cylindrical member 44 of the G3 electrode and to its plate member 46, while a high voltage of from about 20 to about 35 KV is applied to the cylindrical member of the G4 electrode 45 and to its plate member 47. Accordingly, an electrostatic field is generated between the G3 electrode 44 and the G4 electrode 45 due to the difference of the impressed potentials between the G3 electrode 44 and the G4 electrode 45. The trajectories of the three electron beams supplied to the main electron lens are bent by this electrostatic field and eventually, the three electron beams are converged. In this way, a focus is formed on the phosphor screen (not shown) and a beam spot is formed on the screen.
In the conventional in-line type electron gun assembly described above, the cylindrical members of the G3 and G4 electrodes 44, 45 generally have a substantially elliptic cross section of the aperture. When the G3 and G4 electrodes 44, 45 equipped with the cylindrical members having such a substantially elliptic cross section of the aperture are used, however, the major diameter (a diameter in the lateral direction or in a horizontal direction in the drawing) is considerably greater than the minor diameter (a diameter in a longitudinal direction or in a vertical direction in the drawing). For this reason, if the electron beam passage holes formed in the plate members of the electrodes 44 and 45 are found, focusing effects of the three electron beams are different between the vertical direction and the horizontal direction, and the spot shapes on the phosphor screen are expanded in the horizontal direction.
To solve this problem, it has been a customary practice to shape the shapes of the electron beam passage holes disposed in the plate members 46, 47 of the G3 and G4 electrodes into a substantially elliptic shape as in the in-line type electron gun described in the prior art reference, and moreover, to shape the elliptic shape into a longitudinally elongated shape in the vertical direction, contrary to the cross sectional shape of the apertures of the cylindrical members of the G3 and G4 electrodes 44, 45.
According to this structure, the three electron beams that would otherwise tend to expand in the horizontal direction due to the elliptic cross section of the apertures are much more focused in the horizontal direction than in the vertical direction at the portions of the substantially elliptic electron beam passage holes of the plate members 46, 47 of the G3 and G4 electrodes. Accordingly, the section of the three electron beams after passing through the main electron lens becomes generally round and the tendency of the spot shape on the phosphor screen to expand in the horizontal direction is much suppressed.